Light emitting diodes (LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED.
Technological advances over the last decade or more have resulted in LEDs having a smaller footprint, increased emitting efficiency, and reduced cost. LEDs also have an increased operation lifetime compared to other emitters. For example, the operational lifetime of an LED can be over 50,000 hours, while the operational lifetime of an incandescent bulb is approximately 2,000 hours. LEDs can also be more robust than other lights sources and can consume less power. For these and other reasons, LEDs are becoming more popular and are being used in applications that have traditionally been the realm of incandescent, fluorescent, halogen and other emitters.
LEDs are also being used in displays, both big and small. Large screen LED based displays (often referred to as giant screens) are becoming more common in many indoor and outdoor locations, such as at sporting events, race tracks, concerts and in large public areas, such as Times Square in New York City. Many of these displays or screens can be as large as 60 feet tall and 60 feet wide. These screens can include thousands of “pixels” or “pixel modules,” each of which may contain a plurality of LEDs. The pixel modules can use high efficiency and high brightness LEDs that allow the displays to be visible from relatively far away, even in the daytime when viewed in sunlight. The pixel modules can have as few as three LEDs (one red, one green, and one blue) that allow the pixel to emit many different colors of light from combinations of red, green and/or blue light. In the largest jumbo screens, each pixel module can have dozens of LEDs. The pixel modules are arranged in a rectangular grid. For example, a grid can be 640 modules wide and 480 modules high, with the end size of the screen being dependent upon the actual size of the pixel modules.
Conventional LED based displays are controlled by a computer system that accepts an incoming signal (e.g., a television signal) and, based on the particular color needed at the pixel module to form the overall display image, the computer system determines which LED in each of the pixel modules is to emit light and how brightly. A power system can also be included that provides power to each of the pixel modules; the power to each of the LEDs may be modulated so that each LED emits at the desired brightness. Conductors are provided to apply the appropriate power signal to each of the LEDs in the pixel modules.
Present technology utilizes optics and geometries that maximize light extraction from the LED to obtain uniform emission profiles. This usually entails a hemispherical lens coupled to a light emitting element where the optical centers of the lens and the emitting surface are perfectly aligned, and the peak light emission is along the optical axis. Such a configuration may not be advantageous for all situations, however, such as when the LED display is mounted above the viewer's eye level.
Referring now to FIGS. 1 and 2, an exemplary LED display 10 is shown mounted at an elevated point above the eye level of the viewer 12. The viewer 12 is typically positioned below the display 10 and looks up to the display such that the viewer's line of sight 14 to the display 10 is at an angle θ with respect to the display's emission direction 16, which is perpendicular to the display surface. Referring now to FIG. 2, the LED display as shown in FIG. 1 includes a plurality of emitters, such as the LED package 20, which may include an LED 22 mounted in a reflective cup 24 and encased in a generally bullet-shaped encapsulant 26. The peak emission for the LED package 20 is along the package's longitudinal axis 28. FIG. 3 is a polar iso-candelar graph 30 for the LED package 20, showing the peak emission along the emitter's longitudinal axis.
FIG. 1 shows a display comprising a plurality of LED packages 20 emitting with characteristics that display a peak emission directed along the perpendicular direction 16. The intensity profile (Iv) and far field pattern (FFP) peak emission characteristics for the LED display 10 are also perpendicular to the display along the perpendicular axis 16. Because the viewer's line of sight 14 is below perpendicular when the display 10 is mounted at an elevated point, much of the light emitted by the display may not be seen by the viewer and is thus wasted.
One way to reduce the amount of light that is wasted is by mounting the display at an angle to better match the viewer's line of sight 14, but this can require complex and expensive mounting hardware that is difficult to use, particularly for very large displays mounted at high elevations. Efforts have also been made to control the light emission from LED packages by modifying the shape of the encapsulant or lens, but this may require special, costly lens tooling and modified lens fabrication processes. Some systems may utilize secondary optics to alter beam profiles or redirect light patterns to different angles; however, the secondary optics may incur significant losses on the order of 10-12% and add cost to the display system.